Document prefs/config keys

[nicoburns]

Gecko's pref definitions (https://github.com/mozilla-firefox/firefox/blob/main/modules/libpref/init/StaticPrefList.yaml), many of which correspond to Stylo prefs each have a prose description that documents what the pref does. We should similarly document the prefs in the stylo_static_prefs crate (perhaps copying the descriptions from Gecko where appropriate.

[Loirooriol]

#314 lists all possible prefs there, so after that it would just be a matter of adding comments.